1. Field of the Invention
The present invention relates to signal processing. In particular, the present invention relates to applying crest factor reduction techniques to a digitally modulated signal used in communication application.
2. Discussion of the Related Art
The crest factor of a modulated signal is the square root of the signal's peak-to-average power ratio (PAR). Signals with large crest factor are widely used in communication systems. In many 4G/4G communication systems (e.g. WCDMA or LTE), when seen in the frequency domain, the signal band is divided into a number of non-overlapping sub-bands or carrier signals, with each carrier signal having its own multiple-access modulation format. The sampling rate of the signal is typically higher than the Nyquist rate, i.e. the signal's double-side bandwidth. In such systems, crest factor reduction (CFR) improves power efficiency in a wireless transmitter.
In the prior art, various CFR methods have been developed. However, many of these methods require modifications of either the data symbols used or the modulation schemes. Such methods are unsuitable for post-processing multi-carrier modulated signals for CFR, because the data symbols and modulation details are not available for the CFR processor.
One method that is suitable for multi-carrier signals is known as the “windowing method.” However, the windowing method performs poorly due to fundamental drawbacks in the algorithm.
FIG. 13 (a) illustrates the “clip-filter” method for CFR in the prior art. As shown in FIG. 13(a), an input signal is polar clipped in polar clipper 1301. In a polar clipper, without changing the signal phase, the signal is either clipped, if the signal power is greater than a certain threshold, or amplified with unitygain. The clipped signal is then filtered by filter 1302, which is required to have a flat magnitude response within the signal band to avoid changing the carrier-power profile. The flat magnitude response requirement renders the method not suitable for unbalanced multi-carrier signals, because low-power carriers suffer more error-vector-magnitude (EVM) degradation than high-power carriers.
FIG. 13 (b) illustrates a modified clip-filter method in the prior art. This modified clip-filter method, which can be used with unbalanced multi-carrier signals, is described in U.S. Pat. Nos. 7,170,952 and 7,095,798, for example. However, the finite impulse response (FIR) filters required for this method typically involve a large number of taps. Such filters consume high power and require large silicon areas in a conventional FIR design. As the performance of a single-stage clip filter is limited, CFR performance can be improved by cascading multiple clip filter stages. The improvement, however, increases slowly with each additional stage, which makes it difficult to achieve the desired performance under real-world constraints on power consumption and chip area.
Other types of methods that are widely used are the “peak cancellation” methods which use a number of pulse generators to create a cancellation signal. Peak cancellation methods have two drawbacks. First, such methods result in circuits that have high power consumption requirements, due to their computational complexity. For example, the GC1115 integrated circuit marketed by Texas Instrument, Inc. has a peak power consumption of 1.8 watts. Second, such methods result in circuits that have relatively low performance.
Achieving near optimal CFR in arbitrary multi-carrier signals without incurring high computational complexity is highly desired.